The present invention generally relates to a retaining ring used in a polishing head for a polishing process and a method for using and more particularly, relates to a retaining ring that has active edge profile control by using piezoelectric actuator/sensors and a method for using the retaining ring.
Apparatus for polishing thin, flat semiconductor wafers is well-known in the art. Such apparatus normally includes a polishing head which carries a membrane for engaging and forcing a semiconductor wafer against a wetted polishing surface, such as a polishing pad. Either the pad, or the polishing head is rotated and oscillates the wafer over the polishing surface. The polishing head is forced downwardly onto the polishing surface by a pressurized air system or, similar arrangement. The downward force pressing the polishing head against the polishing surface can be adjusted as desired. The polishing head is typically mounted on an elongated pivoting carrier arm, which can move the pressure head between several operative positions. In one operative position, the carrier arm positions a wafer mounted on the pressure head in contact with the polishing pad. In order to remove the wafer from contact with the polishing surface, the carrier arm is first pivoted upwardly to lift the pressure head and wafer from the polishing surface. The carrier arm is then pivoted laterally to move the pressure head and wafer carried by the pressure head to an auxiliary wafer processing station. The auxiliary processing station may include, for example, a station for cleaning the wafer and/or polishing head; a wafer unload station; or, a wafer load station.
More recently, chemical-mechanical polishing (CMP) apparatus has been employed in combination with a pneumatically actuated polishing head. CMP apparatus is used primarily for polishing the front face or device side of a semiconductor wafer during the fabrication of semiconductor devices on the wafer. A wafer is xe2x80x9cplanarizedxe2x80x9d or smoothed one or more times during a fabrication process in order for the top surface of the wafer to be as flat as possible. A wafer is polished by being placed on a carrier and pressed face down onto a polishing pad covered with a slurry of colloidal silica or alumina in de-ionized water.
A schematic of a typical CMP apparatus is shown in FIGS. 1A and 1B. The apparatus 10 for chemical mechanical polishing consists of a rotating wafer holder 14 that holds the wafer 10, the appropriate slurry 24, and a polishing pad 12 which is normally mounted to a rotating table 26 by adhesive means. The polishing pad 12 is applied to the wafer surface 22 at a specific pressure. The chemical mechanical polishing method can be used to provide a planar surface on dielectric layers, on deep and shallow trenches that are filled with polysilicon or oxide, and on various metal films. CMP polishing results from a combination of chemical and mechanical effects. A possible mechanism for the CMP process involves the formation of a chemically altered layer at the surface of the material being polished. The layer is mechanically removed from the underlying bulk material. An altered layer is then regrown on the surface while the process is repeated again. For instance, in metal polishing, a metal oxide may be formed and removed repeatedly.
A polishing pad is typically constructed in two layers overlying a platen with the resilient layer as the outer layer of the pad. The layers are typically made of polyurethane and may include a filler for controlling the dimensional stability of the layers. The polishing pad is usually several times the diameter of a wafer and the wafer is kept off-center on the pad to prevent polishing a non-planar surface onto the wafer. The wafer is also rotated to prevent polishing a taper into the wafer. Although the axis of rotation of the wafer and the axis of rotation of the pad are not collinear, the axes must be parallel. It is known in the art that uniformity in wafer polishing is a function of pressure, velocity and the concentration of chemicals. Edge exclusion is caused, in part, by a non-uniform pressure applied on a wafer. The problem is reduced somewhat through the use of a retaining ring which engages the polishing pad, as shown in the Shendon et al patent.
Referring now to FIG. 1C, wherein an improved CMP head 20, sometimes referred to as a Titan(copyright) head which differs from conventional CMP heads in two major respects is shown. First, the Titan(copyright) head employs a compliant wafer carrier and second, it utilizes a mechanical linkage (not shown) to constrain tilting of the head, thereby maintaining planarity relative to a polishing pad 12, which in turn allows the head to achieve more uniform flatness of the wafer during polishing. The wafer 10 has one entire face thereof engaged by a flexible membrane 16, which biases the opposite face of the wafer 10 into face-to-face engagement with the polishing pad 12. The polishing head and/or pad 12 are moved relative to each other, in a motion to effect polishing of the wafer 10. The polishing head includes an outer retaining ring 14 surrounding the membrane 16, which also engages the polishing pad 12 and functions to hold the head in a steady, desired position during the polishing process. As shown in FIG. 1C, both the retaining ring 14 and the membrane 16 are urged downwardly toward the polishing pad 12 by a linear force indicated by the numeral 18 which is effected through a pneumatic system.
More detailed views of the Titan(copyright) head are shown in FIGS. 2A and 2B. FIG. 2A shows that in a Titan(copyright) head, two separate pressure chambers of a carrier chamber 30 and a membrane chamber 32 are used during a polish process. A carrier pressure 34 exerts on the retaining ring 14, while a membrane pressure 18 translates into wafer backside pressure. The retaining pressure is a function of both the membrane pressure and the carrier pressure, for instance, PRR=2.039 PCARxe2x88x921.908 PMEM.
The operation of the Titan(copyright) head 20 can be shown in FIG. 2B. The Titan(copyright) head 20 picks up a wafer 10 by forming a suction cup with its membrane 16. A pressure is applied to the innertube 28 to force the membrane 16 downwardly onto the wafer 10 to ensure a good seal with the suction cup. A vacuum is thus applied to the membrane 16 to lift the wafer 10. The innertube 28 has little effect on the process because it is pressurized to the same pressure as the membrane chamber 32. During a polishing process, a pressure of approximately 5.2 psi is applied on the retaining ring which is higher than a pressure of approximately 4.5 psi that is applied on the membrane, i.e., on the wafer. The higher pressure applied on the retaining ring ensures that the wafer 10 is always retained in the retaining ring 14. However, after repeated usage, the bottom surface 36 of the retaining ring may be worn out and the wafer 10 may slide out during a polishing process. When such defective condition occurs, the wafer may be severely damaged or even broken.
While the Titan(copyright) head described above is equipped with means for applying a pressure on the retaining ring directly, i.e. the carrier pressure 34, the same pressure is applied across the entire retaining ring. In other words, the pressure applied on the retaining ring cannot be customized at different locations to compensate for uneven polishing on a polishing pad. Moreover, in certain other carrier head for wafers, such as those utilized in a linear chemical mechanical polishing apparatus, there is no provision for applying a pressure on the retaining ring at all.
As described above, production CMP tools are either with (i.e. Applied Materials(trademark) tools) or without (i.e. Lam Research(copyright) tools) active retaining rings on the carrier head. Wafers polished on CMP tools without active retaining rings may suffer from edge peeling due to high stresses incurred at wafer edge caused by pad dynamics. CMP tools with active retaining rings, such as those of Titan(copyright) head, usually employ gas pressure as a control input to control the retaining ring pressure. Due to machining tolerance and the dynamic behavior of the process, such a single-input-single-output system may not be able to provide a uniform ring pressure across the ring throughout a CMP process.
Commercial CMP tools usually use a carrier head to carry or hold wafers during polishing and wafer transfer. A carrier head provides a flat surface to apply a desired pressure on the backside of a wafer to press the front side of the wafer against the polishing pad during the polishing process. The frictional force between the wafer and the polishing pad can slide the wafer away from the carrier head if the wafer is not constrained. A retaining ring therefore provides such a mechanism to constrain the sliding movement of a wafer during the polishing process.
A polishing pad is normally a sheet of porous material which provides pathways for slurry to reach the wafer surface. The pad is not rigid, and therefore, can be compressed by an external force. During the polishing process, the pad has a free thickness where it is away from the wafer, and a compressed (or smaller) thickness underneath the wafer, with a transition region across the wafer edge. As a result, there is a transition in stress at the wafer edge. An xe2x80x9cedge-effectxe2x80x9d caused by such a stress transition can lead to process failures such as edge peeling. It is therefore desirable to use the retaining ring to extend the effective wafer-edge outwardly in the radial direction such that the transition region occurs at the retaining ring edge instead of at the wafer edge. However, if the retaining ring is in contact with the polishing pad, it will be consumed as the wafer is being polished. A mechanism is therefore required to actively control the retaining ring to compensate for the consumption of the ring.
It is therefore an object of the present invention to provide a polishing head for holding a wafer therein during a polishing process that does not have the drawbacks or shortcomings of the conventional polishing heads.
It is another object of the present invention to provide a polishing head for holding a wafer therein during a polishing process that does not produce an edge-effect on the wafer after the polishing is completed.
It is a further object of the present invention to provide a polishing head for holding a wafer therein during a polishing process which does not create defects such as edge peeling on the wafer after the polishing process is completed.
It is another further object of the present invention to provide a polishing head for holding a wafer therein during a polishing process that does not have the edge-effect on the wafer for use either in a rotary or a linear CMP process.
It is still another object of the present invention to provide a polishing head for holding a wafer therein during a polishing process that does not have an edge-effect by using piezoelectric actuator/sensors installed between a carrier head and a retaining ring.
It is yet another object of the present invention to provide a polishing head for holding a wafer therein during a polishing process that does not have the edge-effect on the wafer by using at least three piezoelectric actuator/sensors mounted in-between a carrier head and a retaining ring.
It is still another further object of the present invention to provide a method for improving edge profile on a wafer during a polishing process such that a defect of edge peeling can be avoided.
It is yet another further object of the present invention to provide a method for improving edge profile on a wafer during a polishing process without the edge-effect by mounting at least three piezoelectric actuator/sensors in-between a carrier head and a retaining ring.
In accordance with the present invention, a polishing head for holding a wafer therein during a polishing process that does not produce an edge-effect and a method for using the polishing head are disclosed.
In a preferred embodiment, a polishing head for holding a wafer therein during a polishing process is provided which includes a carrier head that has a recessed peripheral edge portion adapted for engaging top surfaces of at least three piezoelectric actuator/sensors spaced-apart equally along the peripheral edge portion, and a non-recessed center portion that has a bottom surface for intimately engaging a wafer; a retaining ring surrounding the wafer that has a bottom surface in the same horizontal plane of an active surface of the wafer, the retaining ring has a top surface adapted for engaging bottom surfaces of the at least three piezoelectric actuator/sensors; and at least three piezoelectric actuator/sensors mounted in-between the recessed peripheral edge portion of the carrier head and the top surface of the retaining ring.
In the polishing head for holding a wafer therein during a polishing process, the at least three piezoelectric actuator/sensors are each mounted by mounting screws engaging the recessed peripheral edge portion of the carrier head and the top surface of the retaining ring. The at least three piezoelectric actuator/sensors are four piezoelectric actuator/sensors that are mounted equally spaced along the peripheral edge portion. Each of the at least three piezoelectric actuator/sensors exerts an expansion force against the recessed peripheral edge portion of the carrier head and the top surface of the retaining ring when a current is received by the actuator/sensor. The polishing head may further include means for generating air pressure on top of the carrier head for pushing the carrier head downwardly for engaging the wafer to a polishing pad. The carrier head may be used in a rotary-type chemical mechanical polishing apparatus or a linear chemical mechanical polishing apparatus. The polishing head may further include a process controller for receiving force signals on the at least three piezoelectric actuator/sensors, comparing the force signals to pre-stored data, and delivering an electrical current to the at least three piezoelectric actuator/sensors to increase/decrease forces exerted on the retaining ring.
The present invention is further directed to a method for improving edge profile on a wafer during a polishing process which can be carried out by the operating steps of first providing a carrier head that has a recessed peripheral edge portion adapted for engaging top surfaces of at least three piezoelectric actuator/sensors spaced-apart equally along the peripheral edge portion, and a non-recessed center portion that has a bottom surface for intimately engaging a wafer; mounting a retaining ring to surround the wafer, the retaining ring has a bottom surface in the same horizontal plane of an active surface of the wafer, the retaining ring has a top surface adapted for engaging bottom surfaces of the at least three piezoelectric actuator/sensors; mounting at least three piezoelectric actuator/sensors in-between the recessed peripheral edge portion of the carrier head and the top surface of the retaining ring; and flowing a current to and subsequently causing the at least three piezoelectric actuator/sensors to exert a downward force on the retaining ring to improve an edge profile of polishing on the wafer.
The method for improving edge profile on a wafer during a polishing process may further include the step of sensing a force on the retaining ring by the at least three piezoelectric actuator/sensors. The method may further include the step of sensing a force on the retaining ring by the at least three piezoelectric actuator/sensors and sending a signal to a process controller for comparing to a pre-stored datum. The method may further include the steps of sensing a force on the retaining ring by the at least three piezoelectric actuator/sensors, sending a signal to a process controller for comparing to a pre-stored datum, and then flowing a current through the at least three piezoelectric actuator/sensors to increase the force. The method may further include the step of mounting the at least three piezoelectric actuator/sensors by mechanical means, or by screws to the recessed peripheral edge portion of the carrier head and the top surface of the retaining ring. The method may further include the step of mounting four piezoelectric actuator/sensors equally spaced from each other in-between the recessed peripheral edge portion of the carrier head and the top surface of the retaining ring. The method may further include the step of pushing the carrier head downwardly by air pressure to engage the wafer to a polishing pad, or to a polishing pad in a rotary chemical mechanical polishing apparatus, or to a polishing pad in a linear chemical mechanical polishing apparatus.